The Vonnegut

The Vonnegut is a medium-sized cruiser that can carry a combination of passengers and cargo, and is built with modularity in mind. Its open-frame z-pinch fusion torch gives it modest thrust and significant deltaV. Ships like the Vonnegut operate both within the inner system between Earth, Mars, and the Belt, and also between the inner and outer planets.

Some engineering details:

This engine has an Isp of about 3.5e5 seconds (Ve= 3.5e6 m/s) and a thrust of 3.3e5 N. I estimate a dry mass of about 650 tonnes (total length maybe like 72 m or so? Much of that mass is the titanic radiators for dissipating engine heat) Given a mass ratio of 1.5 (325 tonnes of fusion fuel) gives a total deltaV of about 1419 km/s (709 km/s cruise velocity for a one-way leg with refueling at destination; i.e. 2 burns, or 354 km/s cruise velocity for a complete loop with no refueling; 4 burns).

Its modular design means the actual performance depends on its mission and configuration. Actual mass ratios will be whatever is most efficient for a given payload and flight plan.

Also, a world with multiple fusion spacecraft each burning hundreds of tonnes of 3He implies quite a prodigious rate of production, which could only be met either by lots of big tritium breeder reactors (bombard 6Li with neutrons to transmute it to tritium, store tritium and wait for it to decay into 3He) or huge fleets of nuclear skimmer spaceplanes harvesting the atmospheres of Saturn and Uranus.